The present invention relates to a digital signal transmission system for effecting data transmission in a packet form, and more particularly to a digital signal transmission system in which the packet transmission by a called station is facilitated.
As the use of computers has spread and as digital signal processing techniques have progressed, a data comunication technique has moved into the limelight in which a communication system and a data processing system are combined so as to enable information to be processed by on-line processing. As a small-scale communication system, such as a private communication system installed in the precincts of government and public agencies, companies, or the like, such a communication system in a packet form using a communication cable, e.g. a coaxial cable, attracts public attention due to its low cost, high reliability and high transmission efficiency.
In such a packet-form communication system, a number of personal stations are connected to a communication cable for effecting bi-directional transmission to and from a computer disposed in a laboratory or the like so that messages each divided into data blocks of 1,000 to 2,000 bits may be transmitted from each station. Each message is additionally provided with a header containing its designation, running number or the like. In this communication system, control functions are completely distributed to the respective stations and therefore the network per se is a mere passive transmitting medium having no control function. Accordingly, each station begins transmitting a message after it confirms that the transmission line is available. When interference with a packet from another station occurs during the transmitting operation, both the concerned stations stop their transmitting operations. Each of the stations which has stopped its transmitting operation will then try to transmit the message again after a random queuing time.
In such a communication system, any user at any station not only can access one and the same computer but also can utilize any hardware such as a memory or any software such as a program among the hardware and software distributed amongst the plurality of stations. That is, in this communication system, devices such as high speed or high accuracy printers, large scale files, or the like, which have been concentrated at the location of a large central computer in a time sharing system, may be utilized substantially equally by all stations. Thus, it becomes possible not only to economize resources and to improve practical efficiencies but also to develop a large-scaled software system due to the accommodation of programs and data. Further, in such a communication system, there is no priority in using the transmission line among the users or personal stations. Accordingly, there is no master and slave relationship which is often provided in other systems, so that communication may be carried on between any among the connected stations. Further, since the transmission line such as a coaxial cable is constituted by a complete passive circuit, a highly reliable system may be easily provided.
While this communication system has various advantages, but there is a possibility in this system that packets will interfere with each other on the same transmission line since each station may begin transmitting data at any time. Such interference between packets will become significant as the operating efficiency of the transmission line becomes higher.
To solve such a problem, there have been proposed a number of signal transmission systems such as the so-called "Priority Ethernet" and "Reservation Ethernet" Systems. In the former system, the priority of signal transmission of each station is indicated in the preamble portion of the packet so that, in case interference occurs between packets from different stations, one of the packets having higher priority is allowed to be transmitted preferentially. In the latter system, a master station which indicates the operation mode is always set so as to confirm whether each of the other, personal stations has a signal in a reserved mode waiting to be transmitted and the amount of information to be transmitted. As a result, the master station determines in every frame the order of packets to be transmitted by the respective stations so as to allow signals to be transmitted in time division multiplex in the transmitting operation mode.
In the former proposed signal transmission system, however, there is still a problem of variations in signal transmission delay time due to interference among packets having the same priority. Accordingly, this system is not suitable for real time transmission, such as conversational sound communication, in which importance is attached to the real time correspondency between transmitting and receiving operations.
In the latter signal transmission system, however, the above-mentioned inter-station equality is lost because of the existence of the master station. That is, in this system, data communication must be stopped if any failure occurs in the master station, and in this sense the system reliability suffers.
In order to solve this problem, there has been proposed a digital signal transmission system in which real time transmission can be effected without losing the equality among personal stations. In this system, a frame which is cyclically repeated along the time axis is subdivided on the same time axis into a plurality of blocks so that each personal station may be given an opportunity for packet communication within the block. Thus, each station not only may have an equal opportunity to use an empty block but can also effect real time transmission because an opportunity for signal transmission is given periodically in every frame if the station occupies a certain block for a long enough period of time for the signal transmission.
FIG. 1 shows the frame configuration used in the system as mentioned directly above. A frame cyclically repeated on the time axis is constituted by N blocks #1 to #N. Each block is constituted by various bit strings b.sub.1 to b.sub.9 as follows:
b.sub.1 : backward guard time; PA1 b.sub.2 : preamble; PA1 b.sub.3 : start flag; PA1 b.sub.4 : address bit string; PA1 b.sub.5 : control bit string; PA1 b.sub.6 : information bit string; PA1 b.sub.7 : check bit string; PA1 b.sub.8 : end flag; and PA1 b.sub.9 : forward guard time.
The bit strings b.sub.2 to b.sub.5 and b.sub.7 to b.sub.8 are necessary to constitute a packet and are generally referred to as overhead or additional bits. Intervals b.sub.1 and b.sub.9 are generally referred to as guard time. That is, the guard time is an empty bit string for avoiding the situation that adjacent packets overlap with each other due to the delay time which may occur when the packets of each block propagate on the coaxial cable. The backward guard time b.sub.1 is for protecting the rear packet from such an overlap situation, while the forward guard time b.sub.9 is for protecting the forward packet in the same manner. The number of total bits of the backward guard time b.sub.1 and the forward guard time b.sub.9 is represented by g and the guard time (b.sub.1 +b.sub.2) is represented by .tau..sub.g.
In this proposed digital signal transmission system, if no station is sending signals, any station can begin to send out such a frame configuration signal as described above at any time. A station which has first begun to send out a signal onto the communication cable takes the initiative of frame synchronization.
Once the frame synchronization has been established in this manner, all stations can monitor the status of signals transmitted on the communication cable. The user equipment at each station is provided with a memory for indicating the occupation status of the respective blocks in every frame so that the respective blocks are registered in accordance with the received packet signal of each station. When another station sends out a packet signal after the frame synchronization has been established, the station first searches for an empty block in accordance with the contents of the memory, occupies the block to prevent other stations from transmitting in that block, and times its own with the thus occupied block.
The search for empty blocks is effected in the frame immediately preceding the commencement of the packet transmission operation. In this case, the station has to confirm the fact that there are enough blocks available to accommodate the packet to be transmitted. The station must then particularly specify the blocks, and then transmit the packet. However, since any station has an equal opportunity to select any blocks in any frame, there are some cases where a packet transmitted from one station interferes with another packet transmitted from another station. Upon the occurrence of such interference between two packets, each of the concerned stations stops its packet transmission and then, after a lapse of a random time period, tries to retransmit its packet through blocks which are empty at that time. This procedure for transmitting a packet is the same without regard to whether it is by a calling station or a called station.
Interference among packets may rarely occur when the communication volume is low, and even when interference occurs the concerned station or stations will have an early opportunity to occupy other empty blocks. As the amount of communication increases, however, the possibility of packet interference increases, with a consequent increase in the time which elapses before the concerned station, whether it is a calling station or called station, succeeds in transmitting the packet. This increased period of time is quite wasteful and results in a decreased efficiency in the use of the communication cable. Particularly, when a station transmits a large amount of information, such as in the case of picture signal transmission, this station, i.e., the calling station, calls a called station by occupying a number of blocks. In this case, since the number of empty blocks in one frame is significantly decreased, the wasted time which elapses before the called station can occupy empty blocks for its response may be quite long. Further, this proposed system is arranged such that all stations can simultaneously receive one and the same packet and may thus realize a communication system between one station and plural stations (hereinafter referred to as broadcast communication system). In this case, however, a plurality of called stations may simultaneously search for empty blocks for their answer packets, and therefore the possibility of packet interference becomes very high. This results in a problem of extraordinarily lengthened channel connection time required before the initiation of data transfer.